Successful methionine fortification of soy can enhance the nutritional quality of soy proteins. Methionine, however, is highly sensitive to processing conditions, hence the need for kinetic studies to optimize its retention. To establish general kinetic models, it is critical to determine the effect of major parameters involved in methionine degradation, such as temperature, moisture and protein content and initial methionine concentration. The kinetics of methionine degradation as a function of isolated parameters were established by the classical isothermal method and mathematical models describing their relationship were developed. In order to obtain an overall response, models for each independent variable were multiplied together by experimental data and the aid of a computer.In this study, extrusion processing, one of the most common processes for soybean products was considered. Since the influence of temperature on methionine degradation was significant, mathematical models to describe the extrusion behavior of a system in which temperature varies with time and location, was established.Finally, mathematical models describing the kinetics of methionine degradation as a function of all parameters, and mathematical models describing temperature profile of samples during extrusion were combined to predict the retention of methionine. When the simulation was verified by comparing predicted values with experimental data, it was found that the calculated values from this simulation presented the same trends of methionine degradation as those of experimental data with regard to moisture, protein, initial methionine concentration and temperature.